This invention relates generally to apparatus for manufacturing heat exchangers. More particularly, the invention relates to an apparatus, known as a tube expander, for radially expanding the tubes in a plate fin and tube type heat exchanger.
Plate fin and tube type heat exchangers are commonly used in a variety of applications, particularly in air conditioning and refrigeration equipment as well as in engine cooling systems. In such a heat exchanger, a first fluid, typically a refrigerant or an engine coolant, flows through tubes and a second fluid, typically air, flows around the exterior of the tubes. Heat is exchanged between the two fluids through the walls of the tubes. The rate of heat transfer, and therefore the heat transfer performance of the heat exchanger, can be increased by increasing the area of the external surface of the tubes that is exposed to the second fluid. This is typically done by attaching thin metal plates, or plate fins, to the exterior of the tubes. To be effective for transferring heat and also for mechanical reasons, the plate fins must be in firm physical contact with the exterior of the tubes.
FIG. 1 provides further background for the invention. The figure depicts plate fin and tube heat exchanger 10 at an intermediate stage of manufacture. Heat exchanger 10 comprises hairpin tubes 11, tubesheets 12, plate fins 13 and return bends 15. A typical manufacturing process for making heat exchanger 10 includes the steps of bending straight lengths of tubing into hairpin tubes 11, then inserting hairpin tubes 11 through holes in tubesheets 12 and stacks of plate fins 13. In order to allow the passage of the tubes through the tubesheets and plate fins, the holes must be made with a diameter that is slightly larger than the outer diameter of the tubes. After the tubes are inserted into the tubesheets and plate fin stack, the tubes are expanded radially so that the external surface of the tubes firmly contact the fins. The ends of hairpin tubes 11 are expanded to a greater degree than the rest of the tubes in order to form bellmouths 14. Return bends 15 are then inserted into bellmouths 14 to complete a closed flow path for a first fluid to flow through the tubes of heat exchanger 10. The specific configuration of a given heat exchanger may vary. For example, the flow path of the fluid through the tubes may not be a simple series path but the fluid may flow through the heat exchanger in two or more parallel paths. This is accomplished by providing headers instead of return bends. The heat exchanger may also be of the single pass type, with straight rather than hairpin tubes being used so that fluid flows through the heat exchanger from one side to the other. As it applies to the present invention, however, the process of lacing tubes through plate fins having slightly oversize holes, then expanding the tubes to firmly contact the plate fins is common to the manufacture of most types of plate fin and tube heat exchangers.
Although there are a number of means for expanding tubes during the manufacture of a plate fin and tube heat exchanger, the most common way is mechanical, in which an expansion "bullet" is driven through the tube. The bullet is slightly greater in external diameter than the internal diameter of the tube and sized to result in the desired increase in the external diameter of the tube. The bullet is attached to a rod through which the driving force is applied. As the bullet is driven through a tube, the rod is subjected to a compressive force.
FIG. 2 depicts a typical tube expander and provides further background for the present invention. Expander 20 is of the tension type. By this is meant that the tube to be expanded is in tension during the expansion operation. Heat exchanger 10 is positioned relative to expander 20 so that the longitudinal axis of hairpin tube 11 is aligned with the longitudinal axis of expander bullet 23 and expander rod 22. Gripper jaws 32 hold tube 11 in this aligned position during expansion. Bullet 23 is attached to one end of rod 22 with the attachment means, in some applications, allowing for rotation of the bullet about the axis of the rod. The other end of rod 22 is attached to yoke 24. Ram shaft 25 moves yoke 24 laterally in either direction. Shaft 25 in turn is moved by a motive force (not shown) such as a hydraulic cylinder and piston or a motor and a system of gears, cables and blocks or the like. Bearing blocks 31 support shaft 25 and expander rod 22.
In a tube expansion operation, bullet 23, through rod 22, yoke 24 and shaft 25 is first fully retracted (moved to the right in FIG. 2). Tube 11 is then positioned with respect to bullet 23. Gripper laws 32 engage the tube behind bellmouth 14 to hold the tube in place. Then bullet 23 is driven into tube 11, expanding the wall of the tube, increasing the tube's diameter and thus causing a tight mechanical fit with plate fins 13. Bullet 23 is then withdrawn from tube 11 and, if necessary, heat exchanger 10 repositioned so that another tube or tubes may be expanded. If the tubes being expanded are hairpin tubes, then there should be at least two expander rods and bullets driven by yoke 24, as it is desirable to expand both legs of a given hairpin tube at the same time.
Increasingly, the industry is using tubes of smaller diameters in plate fin and tube heat exchangers. In order to pass through these smaller tubes, the bullet rod must be correspondingly smaller in diameter. A smaller rod is, in general, not as strong as a larger rod. It may be necessary to expand tubes in heat exchangers that are as much as three meters or more in length. A long rod with a small diameter is particularly subject to buckling under compressive forces. Buckling of an expander rod may be prevented if the rod is restrained and supported. FIGS. 3A, 3B and 4 depict one prior art apparatus for providing support to prevent buckling. Like reference numerals in FIGS. 3A, 3B and 4 identify like elements. FIG. 3A shows expander 20 with expander rod 22 in the retracted position. Rod 22 is supported by a number of sliding rod support members 41P. FIG. 4 shows another view of one rod support member 41P. Expander rod 22 slideably extends through channel 43P in the upper portion of support member 41P. The lower portion of support member 41P is slideably mounted in guide slot 44P in bed 21 so that member 41P may move laterally relative to bed 21. During an expansion stroke, support member 41P restrains expander rod 22 and prevents it from buckling. As yoke 24 drives rod 22 into a tube, it also causes support members 41P to move and eventually, at the end of the expansion stroke, as shown in FIG. 3B, to collect at the tube mounting end of bed 21. The design of an expander having this type of rod support apparatus must take into account the combined widths W of support members 41P. The overall length of the expander must be at least W longer than a similar expander without the support members. The smaller the diameter of the expander rod used, the more support is required to prevent buckling and thus the more support members are required, resulting in a greater width W. In an expander capable of expanding very small diameter tubes in a very long heat exchanger, the additional length penalty can be quite significant.
What is needed, therefore, is a rod support apparatus for a tube expander that will afford adequate support to the expander rod or rods to prevent buckling but will not require that the expander to be longer because of its presence.